Microcontroller-supervised systems use digital-to-analog converters (DACs) in order to generate analog voltages used for controlling other devices. While commercial DACs generate a voltage as the analog output, in some cases the device to be controlled is essentially current-driven, which means that the behaviour of the controlled device depends on the current injected into or sunk through its input. In the case of these current-driven circuits, additional circuitry is required between the DAC and the controlled device. Such additional circuitry is usually in the form of a voltage-to-current converter, which is also currently referred to as a “transconductance” amplifier.
The simplest approach to voltage-to-current conversion is shown in FIG. 1 and essentially provides for the use of a single, purely passive component such as a resistor. In the diagram of FIG. 1, a resistor R is connected between the output of the DAC and a current-controlled device D, such as a driver unit for a load, such as a semiconductor diode laser source L. The DAC is controlled via a line C by a microcontroller designated M. (While the present invention was developed by paying specific attention to the possible use of circuitry for controlling a laser driver via a microcontroller, reference to this use is not to be construed as limiting the scope of the invention.)
If Vdac designates the voltage output of the DAC and Vin is the voltage at the input of the controlled device D the current Iin input to the device D can be simply expressed as:Iin=(Vdac−Vin)/R.
The arrangement of FIG. 1 has the disadvantage that the resulting current Iin is not stable when the load voltage e.g., the voltage at the input of device D, changes. Additionally, there may be an offset in voltage-to-current response that is a zero current for non-zero voltage and/or vice versa.
Also, there is no positive Iin for positive Vdac if Vdac is less than Vin. If Vin changes (for instance in the presence of a thermal drift in the device to be controlled), Iin changes even if the DAC setting (and thus Vdac) has not changed, which is undesirable in most applications.
An alternative prior art arrangement is shown in FIG. 2, where the same references designate elements identical or equivalent to those already considered in FIG. 1.
The arrangement of FIG. 2 employs a DC operational amplifier A having (1) a positive (non-inverting) input terminal fed with the output voltage Vdac from the DAC and (2) an inverting input terminal fed with the voltage provided by a negative feedback loop comprising a voltage divider connected between the output of the amplifier A and ground. Amplifier A is constructed so the voltage and current at its output terminal is directly proportional to and has the same polarity as the voltage at the amplifier non-inverting input terminal minus the voltage at the amplifier inverting input terminal. The voltage divider in question includes device D to be controlled and resistor R.
In this case, if device D comprising the load of the circuit has an impedance ZL the current Iload flowing through the load can be expressed as:Iload=Vdac/R.
In this case the load current Iload is linear with Vdac. However, the load D floats, that is neither of its terminals is connected to ground. This is seldom true for loads that are active devices such as, for instance, inputs of integrated circuits.
A classic circuit for a ground-terminated load is shown in FIG. 3 wherein voltage Vdac is applied to the inverting input terminal of the amplifier A via first resistor B1. Resistor B4 is connected as a feedback resistor between the amplifier output terminal and the inverting input terminal. The resistors B1 and B4 thus comprise a first voltage divider between the amplifier output and the DAC output. An intermediate point of the divider is connected to the inverting input of the amplifier A. A second voltage divider including resistors B2 and B3 is somewhat similarly associated with the non-inverting input terminal of the amplifier A. Specifically, the resistor B3 is connected between the amplifier output terminal and the non-inverting input terminal while the resistor B2 is connected between the non-inverting input terminal of the amplifier A and ground. Load D is connected in parallel with resistor B2.
The main disadvantage of the circuit of FIG. 3 is that the overall gain is negative. When Vdac is positive, Iload is negative which means that to have a positive Iload, Vdac must be negative. The requirement for Iload and Vdac to have opposite polarities requires a bi-polarity DC power supply. Because most circuits use single, positive-only or negative-only power supply voltages, the circuit of FIG. 3 is usually not feasible.